Cutting inserts having a coating layer deposited on a substrate surface have conventionally been used for different types of purposes. For example, in the cutting tools widely used in cutting of metals, a single or multilayer of coating layers such as a titanium carbide layer (TiC), a titanium nitride (TiN) layer, a titanium carbonitride layer (TiCN), an aluminum oxide (Al2O3) layer and a titanium aluminum nitride (TiAlN) layer are deposited on a surface of a hard substrate composed of cemented carbide, cermet, ceramics or the like. The recent diversification of cutting of materials requires further improvements in fracture resistance and wear resistance.
In order to improve the properties of these coating layers, Patent Document 1 discloses a surface coated tungsten carbide based cutting tool, which is described as exhibiting excellent wear resistance. The cutting tool has, on a surface of a tungsten carbide based cemented carbide substrate, a coating layer constructed from a granular titanium nitride (TiN) layer having a granular crystal structure and an average layer thickness of 0.1 to 1 μm, a titanium carbonitride (TiCN) layer having columnar crystal structure and an average layer thickness of 2 to 15 μm, an aluminum oxide layer (Al2O3) layer having an average layer thickness of 0.5 to 10 μm, and a titanium carbide (TiC) layer having columnar crystal structure and an average layer thickness of 2 to 10 μm.
Patent Document 2 discloses a surface coated tungsten carbide based cemented carbide end mill having a coating layer on a surface of a tungsten carbide based cemented carbide substrate. The coating layer is constructed from a titanium nitride (TiN) layer having a granular crystal structure and an average layer thickness of 0.1 to 1 μm, and a titanium carbonitride (TiCN) layer having an elongated growth crystal structure and an average layer thickness of 0.3 to 3 μm, and further having, on a face and a flank other than a cutting edge, a titanium oxycarbonitride (TiCNO) layer having a granular crystal structure and an average layer thickness of 0.1 to 1 μm, and an aluminum oxide layer (Al2O3) layer having a granular crystal structure and an average layer thickness of 0.1 to 1.5 μm. That is, the Al2O3 layer on the cutting edge is removed to expose the TiCN layer.
Patent Documents 3 and 4 disclose the methods of improving the adhesiveness between the substrate and the coating layer by diffusing the component constituting the substrate into the coating layer.
Specifically, there are disclosed cutting tools obtained by sequentially coating with CVD (chemical vapor deposition) method, on a surface of a substrate composed of tungsten carbide (WC) based cemented carbide containing cobalt (Co), a first layer of titanium carbide (TiC) or titanium nitride (TiN), a second layer of titanium carbonitride (TiCN) containing columnar crystals, a third layer of titanium carbide (TiC), titanium oxycarbide (TiCO) or the like, and a fourth layer of aluminum oxide (Al2O3) layer, and then diffusing tungsten (W) and cobalt (Co) existing in the substrate into the first and second layers or the first to third layers.    Patent Document 1: Japanese Unexamined Patent Publication No. 2000-158207    Patent Document 2: Japanese Unexamined Patent Publication No. 11-229143    Patent Document 3: Japanese Unexamined Patent Publication No. 7-243023    Patent Document 4: Japanese Unexamined Patent Publication No. 8-118105